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Polar ice sheets are melting faster than expected and could push sea levels up as much as 6 metres by 2100, scientists warn in two studies released today.

Based on current warming trends, they say, average temperatures could jump at least 2.5°C by the end of the century, resembling the last great global warming surge 129,000 years ago when seas rose as much.

They used a computer model for future climate predictions and ice sheet simulations to create a picture of the Earth's climate 129,000 years ago.

They also used climate-indicative data from sediments, fossils and ice cores.

They conclude that the melting of ice sheets in Greenland and in the Antarctic today resembles what happened back then, and that the risk of a comparable rise in sea level is high.

"Although the focus of our work is polar, the implications are global. These ice sheets melted before and sea levels rose. The warmth needed isn't that much above present conditions," Otto-Bliesner says.

Dynamic maps of the projected results of the rise in sea levels are available on Overpeck's website.

Call for action

Overpeck says we need to act soon.

"We need to start serious measures to reduce greenhouse gases within the next decade. If we don't do something soon, we're committed to 4-6 metres of sea level rise in the future," he says.

A conservative estimate would call for sea level rises of 1 metre per century, he says.

He cautioned, however, that this estimate assumes the Earth will get only as hot as it did 129,000 years ago when the ice sheets melted.

The earlier ice melt was concentrated in the Northern Hemisphere in the summer months, and was due largely to changes in Earth's orbit, he says.

"The climate warming we're in now is global and it's year-round and it's due to human influences on the climate system," he says.

"That will be more damaging to the ice sheets than the that warming we had 130,000 years ago."